The subject matter disclosed herein relates to ultrasonic transducers, and more particularly to an ultrasonic transducer having improved flow rate measurement accuracy.
Ultrasonic flow meters are used to determine the mean pipe flow rate (Vm) of a variety of fluids (e.g., liquids, gases, multiphase, etc.). Knowledge of the flow rate of the fluid can enable other physical properties or qualities of the fluid to be determined. For example, in some custody-transfer applications, the flow rate can be used to determine the volume (Q) of a fluid (e.g., oil or gas) being transferred from a seller to a buyer through a pipe to determine the costs for the transaction, where the volume is equal to the flow rate multiplied by the cross sectional area (A) of the pipe.
A conventional ultrasonic transducer typically includes a cylindrical housing with the ultrasonic transducer affixed within one end (usually the tip) and an electronics package mounted within the opposing end. An acoustic dampening material typically separates the two to prevent sound waves from reverberating inside the housing. The transducer crystal is fragile and therefore is not normally exposed to the medium being measured. Accordingly, the housing tip is typically hermetically sealed to prevent moisture and contaminants from entering the inner cavity where the transducer is located. Within the inner cavity, the transducer abuts the tip and propagates an ultrasonic signal through the tip material into the medium being measured. The planar face of the transducer crystal is perpendicular to (e.g., normal to) the direction of ultrasonic wave propagation. For applications in which the transducer is utilized as a flow meter, the ultrasonic transducer is then mounted through an access aperture in the fluid conduit.
One noted drawback to this approach is that the ultrasonic signal must first pass through the transducer tip material prior to entering the medium to be measured. The tip material may be quite thick to withstand the pressure of the fluid in the conduit, which in one example is several thousand pounds per square inch (psi). The thick tip material may absorb or otherwise attenuate the ultrasonic signal, causing degraded performance such as decreased signal-to-noise ratio. Furthermore, due to the tip thickness, the probe tip or face must be perpendicular to the direction of propagation. Otherwise, the thickness of the tip material will skew the wave propagation path, leading to measurement errors.
In one type of ultrasonic flow meter employing transit time flow metering, one or more pairs of ultrasonic transducers can be mounted to a pipe (or spool piece attached to a pipeline). Each pair can contain transducers located upstream and downstream from each other forming an ultrasonic path between them. Each transducer, when energized, transmits an ultrasonic signal (e.g., a sound wave) along an ultrasonic path through the flowing fluid that is received by and detected by the other transducer. The path velocity (i.e., path or chord velocity (Vp)) of the fluid averaged along an ultrasonic path can be determined as a function of the differential between (i) the transit time of an ultrasonic signal traveling along the ultrasonic path from the downstream transducer upstream to the upstream transducer against the fluid flow direction, and (2) the transit time of an ultrasonic signal traveling along the ultrasonic path from the upstream transducer downstream to the downstream transducer with the fluid flow direction.
One type of transit time flow meter used in industrial and commercial applications is a flare gas flow meter, which measures the flow rate in a combustible gas that is vented to atmosphere and subsequently burned. Combustible gases are common byproducts in oil refinery operations, oil drilling and exploration, and industrial processes, for example. The safest manner in which to dispose of the combustible gas is to vent it to atmosphere and ignite it. However, environmental regulations sometimes require the flare gas operator to document the amount of combustible gas burned in the atmosphere over a given period of time. The flare gas flow meter allows the operator to measure and document the gas flow in order to remain in compliance with regulations.
Ultrasonic flare gas flow measurement typically utilizes at least one pair of transducers as described above, each transducer being fitted within a probe. Since the flare gas typically flows through the pipe at a very high velocity (e.g., 150 m/s), accurate measurements may be difficult if the probes are spaced far apart, as may be the case in large diameter pipe. Therefore, in some applications each transducer probe protrudes into the flare gas pipe approximately one quarter of the pipe diameter. Each probe protruding into the pipe reduces the separation distance between the probe pair, which allows for a more accurate measurement.
Several problems with this approach arise. One noted problem is that the probes are large and present obstructions to the flow. Due to the dynamic forces in the high velocity flow, the probes may begin to shake or vibrate. The vibration may induce fatigue stress. Also, the velocity of the fluid may tend to bend the probe, either elastically or permanently. In either case, the probe may eventually fail.
Another type of transit time flow meter is a multi-phase flow meter, which measures the flow rate in pipes that contain more than one phase, such as liquid and solid. One example of a multi-phase flow may be found in oil drilling operations, where sand particles are admixed with the liquid oil flowing in the pipe. The sand particles tend to interfere with the ultrasonic waves being transmitted between sensors. One solution to this problem is to insert the probes into the pipe to minimize the distance between the transducers, similar to the flare gas application. One drawback to this approach is that the sand particles erode the probe tips and, over time, cause the probe to fail.
Another type of flow meter is a custody transfer flow meter, which necessitates a very accurate flow measurement. Custody transfer flow meters often measure expensive (and sometimes volatile) fluids such as gasoline. Safety regulations prohibit obstructions in the pipe flow path (such as probes) that could pose an ignition hazard. Therefore, the transducers are typically recess-mounted in the pipe. Due to the geometries involved (e.g., the upstream and downstream cross-mounting) and the requirement that the probe face is perpendicular to the wave propagation, the recess-mounted transducer will form recesses or cavities in the conduit wall. One drawback to this approach is that the flow meters with recessed transducers, such as those found in liquid custody transfer or multiphase flow meters, may experience erosion and blockage in the cavities formed by the recess. In some configurations, the flow velocity passing over the recess forms eddies which, if solid particles such as sand were present in the flow, erode the cavity and conduit. In other configurations, solid particles can settle into the cavities and obstruct the ultrasonic path, leading to erroneous readings.
It would be advantageous to improve flow rate measurement accuracy without inserting the transducer probe into the fluid stream, or recessing the probe from the inner wall of the fluid conduit.